Monomode fibers (i.e. fibers capable of propagating only one electromagnetic mode, the fundamental mode) are usually used for telecommunications, since they have low attenuation and high capacity in terms of information that can be transmitted per unit time.
Propagation characteristics and the number of modes guided in a fiber depend largely on normalized frequency V, which depends in turn on the radius a of the fiber core, the maximum numeric aperture .DELTA. and the operation wavelength .lambda. according to relation: EQU V=2.pi..multidot.a.multidot..DELTA./.lambda..
In particular a value V.sub.0 exists above which (and thus a wavelength .lambda..sub.0 below which) the fiber no longer acts as a monomode fiber because, under these conditions, other modes, besides the fundamental mode, can be propagated.
As the advantages stated depend for the most part on fiber monomodality, precise knowledge of V.sub.0 or .lambda..sub.0 is of interest.
The value of .lambda..sub.0 can be obtained from V.sub.0, determined in turn from the fiber refractive index profile. However the accurate determination of the refractive index profile requires sophisticated measurements and the value obtained can have only limited interest. In fact, the practical use value is an effective value which depends strictly on environmental conditions: hence, the value determined could require empirical corrections. Consequently it would be advantageous to directly measure the effective value of .lambda..sub.0.
An accurate measurement of .lambda..sub.0 should be based on the determination of the power fraction, guided in the fundamental mode by a fiber trunk under test, versus wavelength. The decrease in the power guided in the higher order mode can be quantitatively observed and the cut-off wavelength can be defined as the wavelength at which power fraction guided in the fundamental mode exceeds a predetermined value (e.g. 90% for a fiber trunk 2 m long) so that the power guided in the higher order mode becomes neglectable.
One method of cut-off wavelength measurement based on this principle is described in the paper "Une technique nouvelle pour las mesure de la longueur d'onde de coupure des fibres monomodes" presented by G. Grosso, P. Spano, G. De Marchis, at the 8th ECOC, Cannes, Sept. 21-24, 1982 and published at pages 98-101 of the Conference Proceedings.
This method determines the relative power fraction guided in the different modes through coherence measurements of the electromagnetic field at the output end of the fiber under examination. Accurate values of the cut-off wavelength are obtained, but quite sophisticated equipment is required.
A second method is described by us in the paper "Polarization measurement of cut-off wavelength in monomode fibers", presented at the 9th ECOC, Geneva, Oct. 23-26, 1983, and published at page 193-196 of the Conference Proceedings.
This method takes advantage of the different polarization properties of the fundamental mode with respect to the higher order modes and can be implemented with simple measuring equipment, but the physical conditions of the fiber under test must be accurately controlled. Hence the measurement is time consuming and expensive.